Constraint Parameter for a Longitudinal Surface Notch in a Pipe Submitted to Internal Pressure

Abstract:

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The use of two parameters fracture mechanics criterion as a tool for structural design and analysis has increased significantly in recent years. First, we discuse the elastic solution for the stress distribution at crack tip for two dimensional geometries and particularly constraint as T-stress under various loading conditions. Secondly, using notch fracture mechanics and particularly the Volumetric Method approach, we study the stress distribution at the tip of a notch in pipes submitted to internal pressure. The Notch Stress Intensity Factor Kρ and the effective T-stress are combined into a two-parameter fracture criterion (KIρ-Tef). This approach is then used to quantify the constraint of notch-tip fields for various pipe geometry and loading conditions.

Abstract: The interaction behavior of two non-aligned through-wall cracks in flat plates is
investigated by the finite element method (FEM) under extensive creep condition. The
time-dependent fracture parameter C*-integral along the crack tips are calculated and compared to
the results of a single crack of the same size. For comparison purpose, the interaction of stress
intensity factors (SIFs) is also examined in the study. The results indicated that interaction of
multiple cracks is different between the time- dependent fracture characterized by C*-integral and
linear elastic fracture noted by SIF. The magnifying factors of time-dependent fracture are obviously
larger than that of the linear elastic fracture cases. Therefore, the current re-characterization rule for
multiple cracks developed from linear elastic fracture analysis may lead to a non-conservative result
and should be modified when it is used in the assessment of time dependent failure.

Abstract: The wedge splitting (WS) test is now a promising method to perform stable fracture mechanics tests on concrete-like quasi brittle materials. Fracture parameters, such as fracture toughness and critical crack opening displacement and et.al, are however not easy to determined since formulae available from stress intensity factor manual are restricted to standard specimen geometry. The paper attempts to compute expressions for commonly used fracture parameters for a general wedge splitting specimen. By means of finite element analysis program, test simulation was performed on non-standard wedge splitting specimen with different depth and initiation crack length, and thereafter expressions were proposed for stress intensity factor at the pre-cast tip and crack mouth opening displacement on the load line. Based on the work above, size effect on the unstable fracture toughness and crack extension were investigated, and the consistency of fracture toughness data for various specimen depth as well as initiation crack length is demonstrated. The crack extension is little sensitive to the initiation crack length, it increases with the depth of specimen, which can be explained by the boundary influence of the specimen.

Abstract: The present research examines analytically and experimentally the mode-I and mode-II and mixed mode-Interlaminar fracture toughness of PAN based carbon/epoxy composite. A modified Arcan fixture, well-suited for the study of the behavior of used composite assemblies, was developed in order to focus on the analysis of the fracture behavior of the material. The edge effects are minimized by using an appropriate design of the substrates so that experimental results give reliable data. Also the mode-I and mode-II stress intensity factors were computed for different crack lengths and load orientation angles using finite element analysis. The numerical results show that the modified Arcan specimen is able to provide pure mode-I, pure mode-II and any mixed mode loading conditions. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories. Obtained results indicated that fracture toughness and stress intensity factor for sliding mode enhanced up when the loading angle increased. Mechanism of fracture and toughening were examined by using scanning electron microscopy.

Abstract: Investigation shows that one of the failure modes of HDPE pipe is the crack slowly grows across the thick direction and leads to failure at last. So that it is very important to study the resistance to crack initiation properties of HDPE pipe and its butt-fusion welded joint. The J-integral is applied to character the fracture initiation of a tough polymer for which the concept of linear elastic fracture mechanics (LEFM) are inapplicable for reasonably sized specimen due to extensive plasticity. In this paper, the multiple specimen resistance curve technique was employed for J-integral. The normal single side notched three-point bend (3PB) specimen was used to study the characteristic fracture parameter of high-density polyethylene pipe and its butt-fusion joints at different temperature. Testing results show that values of characteristic fracture parameter are affected by the welding process and experimental temperature respectively. The toughness value of HDPE butt-fusion joint is lowered than that of HDPE pipe. With the temperature decreasing, the toughness value of HDPE pipe and Butt-fusion welded joint decrease. And the same time testing results also show that J-integral can describe the fracture character of high-density polyethylene exactly. Testing results can be used for the engineering design and failure analysis of HDPE pipe.

Abstract: Crack growth path was investigated experimentally, numerically and theoretically using two test specimens subjected to pure mode II loading. The specimens were (a) the center cracked circular disc (CCCD) specimen subjected to diametral compression often called the Brazilian disc and (b) the diagonally loaded square plate (DLSP) specimen containing inclined center crack and subjected to pin loading. A few CCCD and DLSP specimens made of two brittle materials (i.e. marble rock and PMMA) were tested under pure mode II conditions. It was observed that the fracture initiation directions and the fracture paths for the tested specimens differed significantly and grew in two different trajectories. However, it was shown that the experimentally observed fracture paths for both specimens can be predicted theoretically very well by using the incremental crack growth method. Several finite element analyses were performed to simulate the whole fracture trajectories of the tested CCCD and DLSP specimens. At each increment, the direction of fracture initiation for the tip of growing crack was determined using the fracture parameters (i.e. stress intensity factors and T-stress) based on the modified maximum tangential stress (MMTS) criterion. The main difference in the fracture trajectory was found to be related to the magnitude and sign of the fracture parameters (which depend strongly on the specimen geometry and loading configuration) and also the type of tensile or compressive loading in the CCCD and DLSP samples.